When power is transmitted from the crankshaft to a camshaft of an engine by a timing belt or chain, tension is maintained in the belt or chain by a tensioner which comprises a tensioner housing, and a plunger protruding from the housing and spring-biased in the protruding direction. In the process of assembly of the timing transmission of an engine, it is necessary for the plunger to be pushed into the housing prior to installation of the tensioner. Accordingly, tensioners having a protrusion-preventing mechanisms have been proposed. Two such mechanisms are described respectively in Japanese Laid-open Utility Model Publication No. Sho. 56-170347 and Laid-open Patent Publication No. Hei. 7-317854.
FIGS. 27 to 29 show a tensioner 1 having a conventional hook-type, protrusion-preventing mechanism. FIG. 27 is a side elevation showing the hook in the locked condition. FIG. 28 is a side elevation showing the hook in the unlatched condition. FIG. 29 is a front elevation of the tensioner shown in FIG. 27.
The tensioner 1 is attached to an engine by bolts (not shown) inserted through bolt holes 2b (FIG. 29) in a mounting flange 2a provided at the rear end of a tensioner housing 2. A plunger 3, projecting outward from the housing 2, is provided with a toothed rack 3a extending axially on its outer surface. The plunger is slidable in a cylindrical bore in the housing and urged in the protruding direction by a spring (not shown) located inside the bore. A pawl 5 is engageable with the rack 3a to prevent backward movement of the plunger as a result of the force imparted to the plunger by a chain through a chain-engaging tensioning lever (not shown). The tensioner includes a locking pin 4 protruding laterally from the plunger near the front end thereof, and a hook 9, pivoted to the housing 2 on a shaft 8. A notch 9a on the hook is engageable with the locking pin 4, and is formed with an oblique surface 9b, shaped so that a slight movement of the plunger 3 in the retracting direction will effect release of the hook 9 from pin 4. Thus, when the plunger 3 is first moved in the retracting direction by the force exerted by a chain through a tensioner lever, the hook 9 is disengaged from the locking pin 4, and pivots away from the locking pin by gravity.
A problem with the tensioner shown in FIGS. 27-29 is that the unlocking of the hook is dependent on gravity, and consequently the tensioner must be positioned so that the hook swings downward about its pivot axis in order disengage, and remain disengaged from, the locking pin 4.
FIGS. 30 and 31 show a tensioner 11, having another conventional protrusion-preventing mechanism utilizing a hook, FIG. 30 being a plan view and FIG. 31 being a side elevation. Both show the hook in the latched condition.
A plunger 13 is slidable in a cylindrical bore in a housing 12, and biased in the protruding direction by a spring (not shown) provided inside the housing. The locking mechanism comprises a locking pin 14 fixed to the plunger 13, a hook 19 pivotally mounted on the housing 12, and engageable with the locking pin 14. A coiled torsion spring 20 biases the hook 19 toward its unlatched position so that the hook will automatically and permanently disengage from the pin 14 when the plunger is first pushed inward.
The tensioner of FIGS. 30 and 31 is not gravity-dependent, and therefore can be mounted in any attitude. However, it requires an additional part, namely, the coiled torsion spring, and is structurally more complex than the tensioner of FIGS. 27-29 and more difficult to assemble. Moreover, it requires space for assembly of the coiled torsion spring.
Accordingly, objects of the invention are to solve the above-mentioned problems encountered in the assembly and use of conventional protrusion-preventing mechanisms, and to provide a tensioner in which the plunger can be automatically unlocked by a simple, easily assembled, and reliable mechanism.
A preferred tensioner in accordance with the invention comprises a housing and a plunger protruding from the housing. The plunger is movable between a retracted position and a protruding condition and is biased in the protruding direction, preferably by a spring inside the housing. The plunger is temporarily maintained in its retracted position by a locking member comprising a wire spring, which has a locking portion engageable with the plunger when the plunger is in its retracted position. The wire spring, also has two arms extending from the locking portion, the respective arms having front ends pivotally attached to the housing, so that the wire spring is pivotable on the housing from a position in which the locking portion is engaged with the plunger to a position in which the locking portion is disengaged from the plunger. The front ends of the arms are pivoted on axes which are laterally displaced with respect to each other in a direction such that a stress is generated in the wire spring when its locking portion is engaged with the plunger. This stress resiliently urges the wire spring in a direction to disengage the locking portion from the plunger.
The plunger is provided with a detent, preferably in the form of a pin or a groove, having an engagement surface positioned to hold the locking portion in engagement with the plunger when the plunger is in its retracted position, and to allow the locking portion to clear the detent when the plunger is moved farther in a retracting direction from said retracted position, thereby allowing the stress in the wire spring to cause the locking portion to clear the detent.
The wire spring is preferably substantially U-shaped, in which case the front ends of its arms are connected to opposite side walls of the tensioner housing, or substantially L-shaped, in which case the front ends of both arms are connected to the same side of the tensioner housing.
The wire spring always tends to move in a direction such that its stress will decrease. Therefore, when a plunger is moved from its locked position farther in the retracting direction as a result of a pressing force exerted by a chain or belt to which tension is to be applied, the wire spring is released and rotates in the direction in which its stress is decreased, thereby automatically disengaging the plunger.